Can either mailbox be in use during the migration?
Yes. You can be logged into both the Source and Destination mailboxes during a mailbox migration.
If data or folders are removed or moved during the migration, you may experience errors during the migration, or the migration might fail altogether. If this occurs, you may resubmit the mailbox back for migration if it fails, which will continue the migration from where it left off. In addition, after the migration has completed, submit the migration in retry mode to migrate previously failed items.